Mobile terminals using The Global System for Mobile Communication (GSM) as standard for digital wireless communications are called GSM phones. GSM has many more services than just voice telephony. Additional services nowadays allow a great flexibility in where and when GSM phones are used. Today's second-generation GSM networks deliver high quality and secure mobile voice and data services (such as SMS/Text Messaging) with full roaming capabilities across the world.
The Subscriber Identity Module (SIM) inside GSM phones is a smart chip that was originally designed as a secure way to connect individual subscribers to the network. There is an on-going evolution of the SIM into a standardized and secure application platform for GSM and next generation networks.
ETSI (the European Telecommunications Standards Institute) is a not-for-profit organization whose mission is to produce the telecommunications standards for use throughout Europe and beyond. ETSI is also a member in 3GPP which drafts the standards for the third generation networks.
The ETSI standard specification TS 100 977 V8.2.0 (2000-05) describes the interface in mobile terminals between the SIM and the Mobile Equipment (ME) within the digital cellular telecommunications system. The 3GPP specification 31.111 specifies USIM Application Toolkit (USAT) which defines an interface between the Universal ICC (UICC) and the Mobile Equipment (ME), and mandatory ME procedures. USAT is a set of commands and procedures for use during the network operation phase of 3G, in addition to those defined in the 3GPP specification TS 31.101 [13]. The 3GPP specification 51.011 defines the interface between the Subscriber Identity Module SIM)—Mobile Equipment (ME). It defines usage of this interface during the network operation phase of GSM as well as those aspects of the internal organization of the SIM which are related to the network operation phase. Thus the 3GPP specification 51.011 takes in principle over after GSM 11.11,
The logical structure of files in SIM is hierarchical and there are three types of files, i.e. Elementary Files (EF), Dedicated Files (DF) and Master Files (MF), the last mentioned ones being highest in the hierarchy.
Roaming management covers the tools and processes used to control the roaming behavior for the subscribers in a mobile telecommunications network. When a subscriber leaves his home network and receives service from another network, he is said to be roaming.
A subscriber can roam to networks with which the operator of the subscriber's home network has a roaming agreement. Operators tend to have roaming agreements with as many other network operators as possible. However, the case is often that in any given roaming situation, there is one network that is preferred. This could be a network with which the operator has a better agreement and thus pays a lower price for its usage. It could also be a network that belongs to the same operator group as the home operator. The earnings for the subscribers' network usage would thus stay in the same company group if the subscriber could use the preferred networks as much as possible. If the roaming behavior could be efficiently controlled and the subscribers be made to roam into the most preferred network at any given time, large savings could be achieved for the operator. It shall, however, be noted that the opinion on which is the most preferred network may change over time.
Roaming management is thus an important area for the operators to improve the revenue stream. Roaming management gives the operator improved control of what networks its subscribers roam into when the home network can not be reached. Controlling this behavior becomes increasingly important as alliances are formed and the operator needs to manage this behavior on a continuous basis.
Roaming behavior is defined in the telecom standards and is controlled by data that is stored on the SIM (Subscriber Identity Module) card. The handset will modify its roaming behavior based on the contents of the roaming control files on the SIM card.
One such standard, in which the mechanisms that control roaming behavior in the GSM/3G network are defined is TS 23.122 in release 99 version, According to that standard, the roaming behavior is to a large extent controlled by to network selector files on the SIM card of the mobile phone. One of the files is the so-called subscriber-defined PLMN (Public Land Mobile Network) list and the other one is the operator-defined PLMN-list. Older versions of the standard define only one list list of preferred networks.
When the phone shall select a network, it first looks for networks defined in the subscriber defined list. Each network indicated by the subscriber-defined network selector file on the SIM is tried according to the priority order given in that file. If none of the networks listed in that file are possible to select, the phone tries the networks that are listed in the operator controlled network selector file. If still none of these are possible to select, the phone shall randomly choose a network whose signal strength exceeds a threshold value. In reality, this random selection often selects the strongest network. If none of the above methods have succeeded, the phone tries all other networks in order of decreasing signal strength. If the phone receives the information that a network is not allowed for roaming, the phone adds the network to the list of forbidden networks and will not access this network while the phone mains in automatic network selection mode.
However, once the phone is roamed into a network, it will stay on that network. According to the above standard, another file on the SIM shall control how often the phone searches for a higher preference network.
The phone will periodically search for the home network as well as a higher preference non-home network to roam into. Since this periodic network re-selection attempts only consider network of the same country as the network to which the phone is currently registered, the home network will only be attempted when the subscriber is nationally roamed.
The network selection functionality described above is the one defined in TS 23.122 in release 99 version. The functionality is, however, not yet fully implemented in most of the existing phones. This pertains especially to the periodic network re-selection attempts. It is anticipated that future phones will start to support these periodically performed attempts to find a higher preference network.
The fact that most phones do not yet support the periodic network re-selection means that once a phone registers to a non-preferred network, it has a tendency to remain there until something extra-ordinary, like coverage loss, takes place.
Even if the phone is turned off and on, the phone will remember the last network it was roamed onto and will try to go back to that network again by reading the information from the (Location Information) LOCI file on the SIM card. Thus the subscriber remains even longer than needed on the wrong network.
Only if coverage is lost, the phone will search for and switch to another network, which might be a preferred network if that network has coverage.
If no network has coverage, the phone will not be able to register to any network. If the coverage then returns, at the same time, for the previous network and a preferred network, the phone will still tend to register back onto the previous network.
There are a number of mechanisms that are employed today in order to control the roaming behavior as described above. Some of these are described in the following and the weaknesses or short-comings of these are touched upon. All descriptions given assume that the phone performs network selection without user interaction. When network takes places without user interaction, the phone is said to be operating in automatic network selection mode. If network selection is performed by the user, the phone is said to be operating in manual network selection mode. In manual network selection mode, it is in fact the subscriber that manually controls the roaming behavior.
The most basic form of Roaming Management is to define the contents of the roaming control files, i.e. the PLMN list or lists and the network search period, on the SIM cards at the time of issuance. This becomes a static definition of preferred roaming networks.
An improvement to the static model is defined in the GSM 03.48 standard, in which the roaming control files are made updatable over the air. This enables remote update of the roaming control files when price models, agreements and operator constellations change. It is also possible to update the whole subscriber base in this manner. If the operator so chooses, it is possible to limit the group to subscribers that are likely to be roamers.
Due to the reasons described above, the handling of roaming control files as described above is referred to as statistical roaming management since the statistical chance that a subscriber will register with a preferred network is greatly improved if the roaming control files are kept updated. The statistical roaming management thus controls roaming by updating the roaming control files on the SIM. Correctly used, the files can greatly improve the ratio of subscribers that roam into the preferred networks. However, there is never any guarantee that a subscriber will roam onto the preferred network.
The behavior of the statistical roaming management is non-intrusive for the subscriber. The effects are only that a preferred network is chosen initially at network selection or after the defined time period as described above. If no preferred network is present, the subscriber stays on the current network.
If the update of the roaming control files is made before the subscriber enters a roaming area, the phone will select the preferred network if it is present in the area. If the files are updated after the subscriber enters the roaming area, and the subscriber is in the wrong network, the subscriber is connected onto a preferred network only if a loss of coverage of the current network takes place and the preferred network is present.
Due to the above limitations, to the statistical roaming management, solutions for so-called dynamic roaming management exist. In some of these solutions, Dynamic Roaming Management uses active knowledge regarding a subscriber's roaming state and tries to achieve a change in the current roaming situation. In dynamic roaming management, the operator of a subscriber's home network knows when the subscriber roams into a new network. If that is an undesired network, a trigger is generated to a roaming server. The roaming server takes update measures towards the subscriber's SIM to make the phone select a better network. The dynamic roaming management is intrusive to the extent that it attempts to perform an active task of moving the subscriber from one network to another.
In the dynamic roaming management scenario, a special roaming management application (RMA) monitors roaming events in the network, for example by interfacing to the HLR (Home Location Register). When the subscriber roams into a foreign network, the RMA is notified. If the subscriber roamed into a non-preferred network, attempts are made to dynamically move him to a preferred network. This behavior tends to lower the perceived quality of service for the subscriber and might therefore be undesirable from that aspect. Since the operator has a possibility of making money, a dynamic roaming management solution might be employed anyway.
Finally, it shall also be noted that the size of the roaming control file is always going to be limited. That means that it will never be possible to list all the preferred networks in this file.
Operators want to have a more direct control of what network the subscribers roam into even with phones that do no support the periodic network re-selection. Therefore dynamic roaming management is applied.
In the case of dynamic roaming management, the RMA attempts to update the roaming control PLMN-list on the SIM. However, as-described above, this in itself does not make the phone switch networks. When the phone does not support the periodic network re-selection, the measure of only updating the PLMN-list is therefore insufficient. In addition to the PLMN-list update, some other conditions need to be met. As discussed above, such a sufficient condition for switching to the correct network includes losing coverage for the non-preferred network while a preferred network is present. The phone would then switch to the preferred network.
However, this behavior is too non-deterministic for operators that really want to make sure that the phone selects a preferred network.
Therefore, the operators attempt to apply more drastic measures to force the phone to a new network. Such methods include different levels of the refresh command specified in GSM 11.14 possibly in conjunction with the Remote File Management (RFM) application in GSM 03.48. For both of these standard specifications newer versions of the specification with the same basic contents exist.
Such a refresh command can be triggered either as part of the 03.48 RFM or as an instruction to a SIM-based application that in turn triggers the refresh command as specified in GSM 11.14 to the phone. The refresh command has different levels ranging from information to the phone that a specific file on the SIM has been updated to a more or less hard reset of the SIM. The intention of issuing the refresh command is to make the phone search for a preferred network as defined in the roaming control PLMN-lists. This does, however, not work as well as one would hope due to the reasons explained below.
The files on the SIM card and their usage are specified in GSM 11.11. This includes the roaming control files referred to above. In addition to the actual roaming control files, a couple of other files on the SIM affect the network selection behavior. The ones that are subject to usage for dynamic roaming solutions would typically be EFLOCI and EFFPLMN. These Elementary Files (EF) specify LOCation Information and Forbidden Public Land Mobile Networks respectively.
The EFLOCI is used by the phone to remember the Location Area it is registered to. The file contains the network identity and the location area in this network that the phone was last registered to. When the phone starts up, or re-initializes the GSM session, it uses the information in EFLOCI to see where it was last registered. The usage of EFLOCI enables the phone to speed up network selection when starting up. From the information in EFLOCI the phone knows which network it was on before it was turned off. When reestablishing network connection, the phone first attempts to register to the network whose identity is found in EFLOCI. If this fails, the phone starts the rest of the automatic network selection process. This is the fact that causes the tendency for the network selection to behave in the somewhat sticky way described above.
The area of dynamic roaming management is thus troubled by the case that even if the user turns the phone on and off again after updating the roaming control file, EFLOCI will tend to make the phone go back to the non-preferred network.
To overcome this problem, dynamic roaming solutions attempt to update EFLOCI by remote means. The contents of EFLOCI are then replaced by padding data to indicate that the file is empty or actual network data that does not identify the network the phone is registered to.
When EFLOCI has been updated in the above manner, a refresh is issued to make the phone perform a new network selection. The level of refresh can be attempted at different levels to achieve the network search.
The solution described above has shown less successful than desired. The reason is that the phone restores the correct value of the EFLOCI that indicates the current network rather than the value that was remotely written to the file. This re-write of EFLOCI may take place either as a consequence of receiving the remote command to perform the refresh or from the actual execution of the refresh command by the phone. In any case, that nullifies the attempt to clear the EFLOCI and the phone will once again go back to the non-preferred network.
The EFFPLMN specifies networks that are explicitly forbidden for the subscriber to roam into. In applying dynamic roaming management solutions, it is possible to use the EFFPLMN to improve the chances of getting the subscriber to move into the preferred network. The usage also has some severe drawbacks.
A possible usage of EFFPLMN in dynamic roaming is to take the network identity of the non-preferred network that the subscriber is currently roamed into and write that network identity into the EFFPLMN using remote update. When the roaming control PLMN-list is correct and contains the preferred network or networks, the dynamic roaming management solution can issue a refresh action of some severity that has been judged necessary, possibly a hard reset. When the phone re-initializes (i.e. selects a network again), it will find the information in EFLOCI regarding most recent network. However, since that network is also listed in the EFFPLMN, the phone is not allowed to select that network. Therefore, the phone is forced to select another network and will thus attempt the networks listed in the roaming control PLMN-list. Therefore, if a preferred network is available, the phone will select it.
When the RMA detects that the subscriber has roamed onto a preferred network, it can do another remote update and remove the non-preferred network from the EFFPLMN.
Obviously, the handling described concerning the EFFPLMN can cause severe service disruptions to the subscriber. In addition to losing network connection while forced to search for another network, there is a risk that the non-preferred network that the subscriber was roamed to was the only one with coverage in the area. This means that the subscriber will be without service until another network becomes present.
Regardless of the disadvantages of using the EFFPLMN, some operators have still chosen to mechanism since the value of correct roaming has been judged as higher than the negative effects of subjecting the subscriber to service loss. This is clearly not a generally acceptable behavior.